Field of Invention
Provided is an oxidation catalyst for treating combustion exhaust gas, and particularly for reducing ammonia slip associated with a selective catalytic reduction process.
Description of Related Art
Combustion of hydrocarbon fuel produces engine exhaust or flue gas that contains, in large part, relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2). But the exhaust gases also contains, in relatively small part, noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of undesirable components, preferably by a process that, in turn, does not generate other noxious or toxic substances.
NOx, which includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), is a burdensome component to remove from exhaust gas generated by lean burn engines. The reduction of NOx to N2 is particularly problematic in lean burn exhaust gas because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. Notwithstanding, NOx can be reduced by a process commonly known as Selective Catalytic Reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a nitrogenous reducing agent, such as ammonia, into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant such as ammonia is added to an exhaust gas stream prior to contacting the exhaust gas with the SCR catalyst. The reductant is absorbed onto the catalyst and the NO reduction reaction takes place as the gases pass through or over the catalyzed substrate. The chemical equation for stoichiometric SCR reactions using ammonia is:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Most SCR processes utilize a stoichiometric excess of ammonia in order to maximize the conversion of NOx. Unreacted ammonia that passes through the SCR process (also referred to as “ammonia slip”) is undesirable, because the slipped ammonia gas can react with other combustion species and/or negatively impact the atmosphere if released. To reduce ammonia slip, SCR systems can include an ammonia oxidation catalyst (AMOX) (also known as an ammonia slip catalyst (ASC)) downstream of the SCR catalyst.
Catalysts for oxidizing excess ammonia in an exhaust gas are known. For example, U.S. Pat. No. 7,393,511 describes an ammonia oxidation catalyst containing a precious metal, such as platinum, palladium, rhodium, or gold on a support of titania, alumina, silica, zirconia, etc. These catalysts oxidize NH3 to yield N2 and/or secondary NOx+H2O as follows:4NH3+7O2→4NO2+6H2O4NH3+5O2→4NO+6H2O2NH3+2O2→N2O+3H2O4NH3+3O2→2N2+6H2O
To remedy this secondary NOx formation, typical ASCs comprise a top catalyst layer comprising a conventional SCR catalyst and a bottom catalyst layer comprising the ammonia oxidation catalyst. Exhaust gas containing slipped NH3, and little or no NOx, passes through the top SCR layer of the ASC wherein the SCR catalyst stores a portion of the NH3. Another portion of the NH3 continues to permeate through the catalyst until it reaches the bottom layer where it is oxidized into secondary NOx and H2O. The secondary NOx permeates back through the top layer where it reacts with the stored NH3 to produce N2 and H2O.
The top and bottom layers of the ASC are segregated to prevent immediate oxidation of the NH3 which would lead formation of untreated secondary NOx in the exhaust stream. For this reason, the top layer in ASCs is free from noble metals, such as platinum group metals (PGMs). Moreover, the bottom layer containing the PGM-based oxidation catalyst is completely covered by the top layer to prevent untreated secondary NOx from entering the exhaust stream.